Rubber processing oils are used as plasticizers or extenders in the production of rubber. RPO is normally co-produced in the lube oil refining process, including the extraction process. In the extraction process, the raffinate phase is refined to produce the base stock for lube oil blending while the extract phase is further processed to produce the RPO. Conventional techniques produce RPO with polycyclic aromatics (PCA) content of 5 wt % or higher. While the European Union has mandated that the PCA content in RPO (as measured by Method IP346) to be less than 3 wt %, the RPO must still be rich in aromatics in order soften rubber components during processing. In particular, the environmentally approved RPO must exhibit a total aromatics (TA) content of more than 50 wt %, a PCA of less than 3 wt %, an aniline point that is lower than 80° C., a kinematic viscosity from 15 to 30 mm2/s at 100° C., and a flash point that is higher than 250° C.
Maintaining RPO quality while reducing its PCA content to comply with the new environmental regulations has been the goal of intense research. Techniques to reduce the PCA such as by selecting suitable feedstocks for blending or employing additional processing to produce acceptable RPO are described, for example, in U.S. Pat. No. 7,186,876 to Manton et al., EP 0 417 980 A1 to Glenz, U.S. Pat. No. 5,846,405 to Aldous et al., U.S. Pat. application. No. 2005/0272850 to Jois et al, U.S. Pat. No. 6,878,263 to Kaimai et al., U.S. Pat. application. No. 2009/0020453 to Tanaka et al., and U.S. Pat. application. No. 2001/0045377 to Morishima et al. These techniques are not completely satisfactory because the RPOs produced have high PCA contents and/or high aniline points or the processes require stringent operating conditions and/or complex, expensive equipment.